A contone printer uses a combination of dithering or halftoning and printing at different levels of intensity to produce different colors and different shades of lightness and darkness. Contone printers can lay down ink at only a few different levels of intensity (usually only two, ink or no ink). To produce the full range of colors that can be perceived by the human eye, contone printers must also use dithering techniques. Dithering or halftoning creates the illusion of new colors and shades by varying the pattern of black and white dots.
Each pixel value of an array of contone pixels within an area is compared to one of a set of preselected thresholds stored in a repetitive pattern as a halftone matrix or halftone cell. A number of different threshold patterns may be used to describe a halftone cell or dot. These threshold patterns indicate the order in which pixels are switched from white to black to form dots of varying gray density. Typically, a threshold pattern can be classified as either a disperse dot or a cluster dot.
Color halftoning has typically used a dot-on-dot method with rotated halftone screens. The screen pattern for each ink color is rotated at a different angle from the others, with angles chosen to minimize moiré. Much effort has been invested in finding compatible halftone screen patterns, frequencies and angles. The dot-on-dot method achieves different colors by printing one color on top of another. An alternative is a dot-off-dot halftone. Dot-off-dot printing produces different colors by the placement of the dots in side by side relationship. Dot-off-dot methods use a single basic halftone structure and thus eliminate the problem of finding a compatible screen set. They also typically produce a larger color gamut, which is desirable. One problem, however, is that the dot-off-dot approach is sensitive to printer registration. Another issue is a method for creating a desirable clustered-dot screen pattern when there are more than two inks.
There are printing technologies where dot-off-dot methods are practical. Inkjet markers can have excellent registration and can support dispersed dot patterns. Highlight printing using tri-level xerography has perfect registration and uses only two inks. However, other technologies may find dot-off-dot an option in the future as printer registration is improved by better feedback/control methods and/or by electronic registration. Even when the marking technology does support the dot-off-dot approach, there is still the question of how to design a halftone pattern that can be used by three, four or even more colorants. Each colorant must be able to grow to cover the cell area as a clustered dot, yet the overlap must be minimized (the separation maximized).
In printing multi-colored halftoned images it would be desirable to construct the halftone dots with minimal overlap between colors since this gives the largest color gamut. It would also be desirable to separate the colored dots as much as possible to minimize the effects of misregistration. Using a single halftone growth pattern is needed, both to avoid overlap of the dots and to eliminate any moiré effects. Some printing technologies require well formed, compact clustered dots, and a method of achieving these goals for clustered dots when more than two colors are used.